1. Field of the Invention
The present invention relates to a backlight for a liquid crystal display device.
2. Description of the Prior Arts
A liquid crystal display device has recently been used as a display device for a wide variety of electronic devices such as a television set, portable terminal, personal computer, electronic notebook and camera-integrated VTR, since it has a thin size, light weight and reduced power consumption. Different from a Brown tube or plasma display, the liquid crystal display device does not emit light but it displays an image or the like by controlling a quantity of light incident from the outside. In such a liquid crystal display device, a backlight for a liquid crystal display device for lighting the liquid crystal display device is provided on its backside (see Japanese Unexamined Patent Application No. 6-242439).
In a circumstance that reduced power consumption has been regarded as important in the aforesaid electronic devices, it has been said that the backlight for a liquid crystal display device holds about a half of the total power consumption of the aforesaid electronic devices. Consequently, it is important to reduce power consumption of the backlight for a liquid crystal display device. On the other hand, a demand for reducing power consumption of the backlight for a liquid crystal display device has been more and more increased when a size of a display screen of a liquid crystal television set, for example, has been increased. In view of the technical background of the backlight for a liquid crystal display device, the inventor has intended to reduce power consumption of the backlight for a liquid crystal display device and simultaneously has made an earnest study of a backlight for a liquid crystal display device that can light a liquid crystal display device with high intensity.
A backlight for a liquid crystal display device uses a cold cathode tube. Its required brightness is 10,000 cd/m2 in a typical backlight for a liquid crystal display device. In order to obtain this required brightness regardless of the screen size, an arrangement of a so-called directly-below type has to be adopted wherein an arc tube is arranged immediately below the backside face of the liquid crystal display device, since the required brightness cannot be obtained by a so-called edge-light type backlight wherein an arc tube (cold cathode tube) is arranged at the side face of the liquid crystal display device for lighting the liquid crystal display device by using a reflection plate, light-guiding plate, diffusion sheet, prism sheet or the like. FIG. 7 and FIG. 8 respectively show a backlight of edge-light type and directly-below type. In FIG. 7, numeral 9 denotes a reflector, 10 a cold cathode tube (arc tube), 11 a reflection plate, 12 a light-guiding plate, 13 a diffusion sheet and 14 a prism sheet. In FIG. 8, numeral 15 denotes cold cathode tubes, 16 a reflection plate, 17 a diffusion plate, 18 a diffusion sheet and 19 a prism sheet.
In the directly-below arrangement, plural straight-tube type arc tubes are arranged in lines or arc tubes are meanderingly arranged. However, even in the directly-below type backlight, the luminous brightness of the arc tube itself is low, so that a large quantity of luminous power is still consumed in order to obtain the required brightness. Further, it is necessary to increase the installation density of the arc tube or increase its number to be arranged in order to obtain the required brightness involved with the increased size of the liquid crystal television set. Therefore, power consumption has been rapidly increased.
Moreover, there are many components to compose the backlight since it is required to provide the diffusion sheet or diffusion plate for diffusing the emission of light in both the edge-light type backlight and the directly-below type backlight, thereby entailing a disadvantage of increasing production cost.
The present invention is accomplished in view of the technical background of the conventional backlight for a liquid crystal display device. The subject that should be solved by the present invention is to provide with low cost a backlight for a flat panel liquid crystal display device wherein the number of components such as a costly diffusion sheet required in the edge-light type or directly-below type is reduced, to thereby be capable of lighting the liquid crystal display device in a plane manner without non-uniformity in illumination with reduced power consumption and high luminous brightness.